Overmolding

Stamp forming can be used for high volume production of continuous fiber reinforced thermoplastic parts with excellent mechanical properties. The process is, however, currently limited to relatively simple shell-like structures. The overmolding technology potentially overcomes this limitation by combining two processes. Firstly, a continuous fiber reinforced thermoplastic composite blank is shaped in a stamp forming step. Secondly, additional features, such as ribs or inserts, are over-injection molded onto the stamp-formed part. Of course, the two processes can be performed in the same tooling and equipment. This way, overmolding combines the high specific strength and stiffness of the continuous fibers with the design freedom and flexibility of a (short fiber reinforced) injection molding grade. Moreover, the process can be easily automated and is well suited for high volume production.

Background/Introduction

The overmolding process combines the forming of a thermoplastic composite with an injection or compression molding process. The process allows for complex parts with high structural performance due to the continuous fibers that, in the ideal case, are positioned along the load paths in the part. Further advantages are the high level of function integration, net shape processing, edge finishing and its potential for large series production in an automated process. However, the availability of design tools for the manufacturing of complex parts using the overmolding process is limited. Here, these tools are being developed with the purpose of reducing the development time and cost of a new part.

Approach

For a precise prediction of the interface strength and the warpage, the following steps need to be taken into account simultaneously:

The fiber reorientation and accompanying thickness change during forming are needed for a proper design of the injection mold cavity;

Both process-induced residual stresses caused by the forming and the injection step are required for a warpage simulation and subsequent compensation of the tool geometry, placement of injection points and the thermal management;

The formation of a good bond between the injected polymer and the composite part, which determines the ability to transfer stress between the injected structure and the composite part, needs to be known along the entire flow pattern of the injected polymer melt.

The accumulation of the process-induced stress caused by shrinkage is needed for an accurate strength prediction.

TPRC, the ThermoPlastic composites Research Center in the Netherlands, is an open innovation, research- and development center that aims for thermoplastic composites for a broad range of end use markets.